Wood is one of the oldest materials known to man. Its ready availability in most parts of the world, coupled with the ease with which it can be cut and fabricated into useful articles makes it well suited for a variety of applications. However, because wood is a cellulosic material it is susceptible to decay and deterioration over any extended period of time. As a result, numerous methods have been proposed for treating wood to improve its durability, particularly when used under conditions where the wood is exposed to moisture or biological attack.
Applications of wood such as its use in manufacturing railroad ties, utility poles, fence posts, and the like have created substantial incentives for industry to develop effective and commercially practical methods of treating the wood to extend its useful life (i.e. wood preservation). Various techniques of wood preservation have been used including surface coating, surface treating, and pressure penetration of the wood with a variety of chemicals such as creosote, pentachlorophenol, and the like.
For example, halogenated phenols such as pentachlorophenol are often used to treat wooden poles (such as utility poles). When used for this purpose, it is common to dissolve the pentachlorophenol in a hydrocarbon oil, pressure treat the poles, and then evaporate the excess oil. Such a process is relatively expensive, but is justified by the wood preservative properties of the pentachlorophenol. However, the treated poles tend to "bleed" their treating solution into the surrounding soil when the poles are installed in the ground. This draining or leaching effect is more visible on the sunny side of utility poles which drain or bleed faster because the heat from the sun warms the treating solution and reduces its viscosity. In an effort to reduce treating costs, water is sometimes substituted for the hydrocarbon oil, and the pentachlorophenol is used in one of its salt forms to provide water solubility. However, this approach has a number of disadvantages and is less preferred than the oil treatment.
Unfortunately, most of the present treating processes (e.g. those using creosote, various copper and mercury compounds, or pentachlorophenol), even when operated under significant pressure conditions, do not result in deep impregnation of the wood. Thus, the wood being treated with any chemical is typically treated to only a shallow depth and, in the case of logs or poles very often the degree of penetration does not extend throughout all of the soft or sapwood, much less penetrate the heartwood. Except for one known process, none of the known commercial processes for preserving wood by chemical treatment involves penetrating the wood poles or logs more than perhaps a few millimeters into the heartwood. The one known exception is the process of U.S. Pat. No. 3,968,276.
According to U.S. Pat. No. 3,968,276 wood (logs or dimension lumber) is preserved by impregnating the wood with chemicals which penetrate substantially to the core or center of the wood being treated. In the case of logs or poles, this penetration can be deep within the heartwood and may be substantially complete. This is accomplished by enclosing the wood within a chamber, sealing the chamber, evacuating the chamber slowly so that the wood is exposed to a pressure below atmospheric pressure, introducing into the chamber a sufficient amount of an aqueous solution of resin prepolymer to immerse the wood and then maintaining the vacuum over the immersed wood for a period of time both to allow the removal of moisture and volatiles from the wood and to permit the treating solution to enter into the wood. Thereafter, an additional amount of the resin prepolymer solution is introduced into the chamber (if needed) and the pressure on the system is increased to superatmospheric pressure to thereby cause an even deeper penetration of the treating solution into the wood. The vacuum and pressure cycle may be repeated one or more times (e.g. 1 to 30 or more cycles). When the treatment is completed to the desired extent, the treated wood is recovered from the chamber.
The process just described is extremely effective and achieves substantially greater penetration of the wood than other commonly practiced treating processes.
It would be beneficial to industry if common wood preservatives such as pentachlorophenol could be introduced deep within the wood being treated and if the bleeding or leaching of the pentachlorophenol could be reduced.